Fibre reflector

ABSTRACT

An air bubble (23) in the core (3) of an optical fiber (1, 1&#39;) constitutes a broad band reflector to light waves propagated in the fiber. A fiber end (3) is first stripped and cut in the conventional way, after which in the end surface, a pit (13) in the core region is produced by etching. The fiber end is therefor treated with e.g. diluted hydrofluoric acid, that etches the higher doped core (3) more rapidly than the cladding (5). The etched fiber end (1) is then welded to a standard fiber so that a splice (21) is obtained where the desired air bubble (23) is enclosed in the fiber core. The magnitude of the reflectance of the produced reflector can then be changed by repeated heating steps, in the same way as in welding, of the splice (21) and/or by filling the bubble (23) with another medium than air or by coating one of its walls with a suitable material, e.g. a metal.

TECHNICAL FIELD

The present invention relates to a method of producing an optical fibrereflector, a fibre reflector produced by means of the method and a useof the fibre reflector for verifying the function of an optical network.

BACKGROUND

Internal reflections in optical fibres have several applications, suchas for sensor purposes and for verifying the function of a fibre opticalnetwork. Light transmitted in an optical fibre is reflected at thereflector and the reflected light can be detected.

Optical reflectors for generating internal reflections in optical fibresand methods for producing such reflectors are previously known from U.S.Pat. Nos. 4,892,388 and 4,400,056. However, the prior methods are rathercomplicated and there is thus a need for providing simple productionmethods for reflectors in optical fibres.

In the patent U.S. Pat. No. 5,210,801 optical components are disclosedconstructed of flat wave guides or wave guides having rectangularcross-sections, where a cavity is arranged located adjacent a wave guidecore, however never so located that the material of the wave guide coreis located directly at the cavity. The production of the components ismade by means of the conventional, process technological, relativelycomplicated and costly methods which are used for manufacturingelectronic integrated circuits and optical planar circuits.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a simple method forproducing internal reflections in optical fibres.

It is also an object of the invention to provide an optical fibreprovided with an internal reflector for obtaining internal reflectionsin the optical fibre.

The objects mentioned above are achieved by the invention, the detailedcharacteristics of which are described below.

Thus, in a controlled way an air bubble, i.e. a small cavity, which inthe main case is filled with air, is introduced in the core of anoptical fibre by means of an etching or a welding procedure and therebya broad band fibre reflector is obtained. The fibre end is firststripped, i.e. its exterior protective layer is removed, and is then cutin the conventional way for achieving an essentially flat end surface,that is located essentially perpendicularly to the longitudinaldirection of the fibre end, after which, in the end surface thus formed,a pit or recess is made in the core region, i.e. in the region where thefibre core ends in the end surface. The pit can be produced by means ofsome mechanical method such as grinding or by treating the end surfacewith a laser beam for removing material at the suitable position.However, a chemical treatment is preferred, wherein the fibre end isetched with e.g. diluted hydrofluoric acid or a mixture thereof withsimilar substances such as ammonium fluoride. A recess is therebyproduced, since the higher doped core is etched more rapidly than thecladding. The etched fibre end is then joined or spliced, in thepreferred case welded, to a standard fibre having, in the preferredcase, an essentially flat end surface, which is cut in the usual way andis essentially perpendicularly to the longitudinal direction of thisfibre end, which results in an air bubble enclosed in the core. Themagnitude of the reflectance can be changed by repeated heatingoperations in the same way as in welding over the spliced or jointregion and/or by filling the bubble with another medium than air or bycoating one of its walls with a suitable material, e.g. a metal.

Generally, an optical fibre has a fibre core and a cladding surroundingthe core. In order to produce a reflector, a cavity is arranged in theoptical fibre, which is in particular arranged in the fibre core. Thecavity is generally completely surrounded by material of the fibre andis located so close to the core, that light which is introduced into andpropagates along the fibre, will be disturbed significantly by thecavity, so that it is partly reflected. It is obtained by the fact thatthe fibre core material extends up to the cavity and in particular thecavity can be essentially completely enclosed or completely surroundedby material in the core. For a production according to what has beendiscussed above the cavity will have substantially a lens-shape oressentially the shape of a flattened ellipsoidic body, having twoopposite, large arched or curved surfaces. The fibre core materialextends then up to at least the central part of these large surfaces andalso to nearly all of these surfaces except possibly a small, exteriormarginal area.

In the cavity a substance can be provided that increases the reflectivecapability of the cavity to light waves propagating in the opticalfibre. Such a substance can in particular have a refractive indexdifferent from the refractive index of the material surrounding thecavity and the optical fibre or it can be a metal material. Thesubstance can, as for metal materials, have substantially solid form andthen be deposited as a layer on only one surface in the cavity, on oneof the large surfaces according to what has been said above, orgenerally only on one surface which is directed towards or is located atthe wave guide core or only one surface which is substantially directedin one direction, in the longitudinal direction of the fibre.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to notlimiting embodiments and the accompanying drawings in which:

FIG. 1 shows a fibre end having an etched end surface,

FIG. 2 shows a spliced fibre having an enclosed cavity,

FIG. 3 illustrates the treatment of the end surface of the fibre,

FIG. 4 shows schematically splicing of optical fibres,

FIG. 5 shows a section of a coated end of an optical fibre,

FIG. 6 shows a section of the coated end according to FIG. 5, where thecoating has been partly removed,

FIG. 7 shows internal reflections in optical fibres used for verifyingthe function of an optical network.

DETAILED DESCRIPTION

In FIG. 1 the end of an optical fibre 1 is shown. It comprises asconventional a fibre core 3, a cladding 5 surrounding the core and anexterior protective layer 7. For conventional quartz glass fibres usedfor telecommunication, the core 3 and the cladding 5 consist of quartzglass having refractive indexes of different magnitudes, produced bysuitable dopings, and the exterior protective layer is made of somepolymer. In the production of an internal reflector first the exteriorprotective layer 7 is removed over a suitable distance from one end ofthe fibre. The end of the fibre is then submerged in a vessel 9, seeFIG. 3, containing an etching liquid in the shape of hydrofluoric aciddiluted with water, possibly mixed with ammonium fluoride, NH₄ F, duringmixing by means of a mixer shown at 11. When the glass in the fibre isetched by means of one of the liquids mentioned, the higher doped fibrecore 3 will be etched more rapidly than the cladding 5 which produces apit or recess 13 in the end surface of the fibre, see FIG. 1. The pit 13can be more or less well localized to the very fibre core 3, dependingon the type of fibre. A continuous transition in the end surface betweenthe region of the fibre core and the cladding region is however alwaysobtained, also for step index fibres.

The end surface of the fibre 1 comprising the pit or recess 13 withinthe core region is then spliced to an end surface of another opticalfibre. The splicing process is advantageously made as a fusionedwelding, where the heating is accomplished in some way, such as by meansof an electrical arch, which will be described hereinafter, by means ofa laser beam, hydrogen gas flame, etc. Also a completely mechanicalsplice is conceivable, such as by means of aligned V-grooves in asuitable fixture.

The fibre 1 comprising the pit 13 in one end surface is thus inserted asone of the fibres in a fibre welding device, generally designed 15, somecentral parts of which being illustrated in FIG. 4. The fibre portion 1is then attached in a guide in one chuck 17, which in the weldingmachine 15 is placed on some support 19. In the opposite chuck 17' asimilar fibre 1' is placed, which in the conventional way is straightlycut but has not been treated for producing a pit 13. In the fibrewelding machine 15 the electrodes 20 are activated by supplying highvoltage thereto, whereby an electric arc is formed therebetween. Itproduces a heat development, so that the ends of the two fibres 1 and 1'melt, these ends being located, during the welding stage, rather closeto each other. When the fibre ends have melted sufficiently, they can bepressed somewhat to each other, so that the melted material flowstogether and the fibre ends are welded to each other, after which theelectric arch is turned off, by interrupting the voltage supply to theelectrodes 20, and the splice produced is finally allowed to cool.

A composite fibre is obtained, as is illustrated in FIG. 2, comprisingthe ends of the two fibres 1 and 1' welded to each other at the weldingregion 21. Owing to the pit 13 in the end of one of the fibres, afterthe welding a small air bubble or cavity 23 is formed in the fibre corein the splice-welded fibre. As has been possible to observe up to now,this bubble is completely enclosed in the fibre core and is in any casesubstantially always located within the core region, i.e. within thecentral region of the fibre where the refractive index varies and isdifferent from the refractive index in the main portion of the fibrecladding 5. This air bubble 23 is a discontinuity in the fibre core andoperates as a reflector or mirror for an electromagnetic wavetransmitted in the optical fibre.

The resulting bubble 23 must have such a size that it affects theradiation field of a light wave that propagates along the optical fibre.It means that its dimension generally should be of the same magnitude oforder as the diameter of the fibre core and that its smallestcross-measure, usually equal to its width as seen in the longitudinaldirection of the fibre, e.g. must exceed 1/5 or 1/10 of the corediameter. Further the cavity should not disturb the light wave tooextensively, since in most cases it is desirable that the light wavealso continues forwards in the fibre, even if it has a reducedintensity.

The size of the resulting bubble 23 and thereby the magnitude of areflection against the bubble 23 can to a certain extent be varied,after having welded the fibres 1 and 1' to each other, by performingfurther heating steps to the melting state or near melting state of thewelded splice, e.g. by means of an electrical arc in the welding device15. In the further heating operations also the dimensions of theenclosed bubble is reduced more and more and then also the reflectanceor alternatively the attenuation.

An air bubble 23 produced in that manner at the core region causesreflections for light having different wave lengths. It has up to nowbeen possible to vary the reflectance from -20 dB and downwards by avariation of the production conditions.

A method of increasing the reflectance owing to the enclosed air bubble23 can be to fill, before splicing the fibre ends, the etched pit 13with substance other than air. An example thereof is illustrated inFIGS. 5 and 6. In FIG. 5 thus a section is shown of a fibre end havingan etched pit 13, where a metal layer 25 is deposited all over the endof the fibre, i.e. in particular over the perpendicularly cut, flat endsurface and over the pit 13. This layer 25 can be a metal layer whichhas been coated by means of some deposition method such as evaporation.Thereafter most of this layer 25 is removed over the end surface of thefibre, as is illustrated of the sectional view of FIG. 6. If the endsurface of the fibre is thus polished in a suitable way, metal materialwill remain in the pit 13 but not on the other portions of the endsurface. Material 27 that is left in the pit 13 will increase thereflections in the finished fibre, which is then, in the same way asabove, obtained by welding the end of the fibre 1 to another opticalfibre having no particularly treated end surface.

In FIG. 7 it is schematically shown how an optical fibre 1, 1' producedin this way comprising an enclosed air bubble or discontinuity can beused for checking the operation of a fibre optical network, compare theInternational Patent Application having publication No. WO 90/06498. Acontrol and monitoring unit 29, which e.g. can utilize OTDR (OpticalTime Domain Reflectometry), is in a suitable way coupled to a main line31 in an optical fibre network. The main line 31 is further connected toan optical coupler 33, from which individual branch lines 39 extend. Ineach branch line 35 a welded assembly is connected consisting of splicedfibres 1, 1' comprising a reflector 21 made in the splice. The controland monitoring unit 29 emits light pulses into the main line 31, whichare distributed further to the branch lines 35. The emitted light isrefractive against the reflectors in the fibre assembles 1, 1' and thecontrol and monitoring unit 29 detects the returning light. If thereflectors 21 are located at different optical distances from thecontrol and monitoring unit 29 the reflections from the different branchlines 35 can be distinguished. If an interrupt is obtained in somebranch line 35, the earlier reflection from the reflector 21 in thisbranch line will cease and thereby it is possible to decide which one ofthe connected branch lines 35 is faulty or incorrect. In this case thecontrol and monitoring unit 29 can output an alarm or suitable signalfor indicating that the branch line in question has been detected to bein error.

What is claimed is:
 1. An optical fiber for telecommunicationcomprising:a fiber core, a cladding surrounding the fiber core, and areflector formed by a cavity in the optical fiber for reflecting lightpropagating in a first direction along the optical fiber, the reflectedlight propagating along the optical fiber in a second direction oppositethe first direction, wherein a substance is disposed in the cavity forincreasing the reflective capability of the cavity for light wavespropagating in the optical fiber.
 2. The optical fiber of claim 1,wherein the cavity is located within the fiber core of the opticalfiber.
 3. The optical fiber of claim 1, wherein the cavity is shaped tohave one of substantially a lens shape and substantially a flattenedellipsoidal body shape, having two opposing large curved or archedsurfaces, and wherein material in the fiber core extends up to at leasta central part of one of the opposing large surfaces.
 4. The opticalfiber of claim 1, wherein the substance has a refractive index differentfrom the refractive index of material in the optical fiber enclosing thecavity.
 5. The optical fiber of claim 1, wherein the substance hassubstantially a solid shape.
 6. The optical fiber of claim 5, whereinthe substance is coated on only one surface in the cavity.
 7. Theoptical fiber of claim 1, wherein the substance is a metal.
 8. A methodof producing a reflector in an optical fiber for telecommunicationhaving a core enclosed by a cladding, the reflector reflecting lightpropagating along the optical fiber in a first direction to a seconddirection opposite the first direction, the method comprising the stepsof:forming a recess on a first end surface of a first piece of anoptical fiber, the first end surface being substantially perpendicularto an axis of the first piece of optical fiber, and joining the firstend surface to a second end surface of a second piece of an opticalfiber, the second end surface being substantially perpendicular to anaxis of the second piece of optical fiber wherein a cavity enclosed bythe first end surface and the second end surface is provided, the cavityforming the reflector.
 9. The method of claim 8, wherein the first endsurface is joined to the second end surface by melting the end surfaces.10. The method of claim 8, wherein the first end surface is joined tothe second end surface by welding the first end surface to the secondend surface.
 11. The method of claim 8, wherein the step of forming therecess on the first end surface includes forming the recess only withina region that corresponds to an end surface of the fiber core.
 12. Themethod of claim 8, wherein the step of forming the recess on the firstend surface includes chemically treating the first end surface.
 13. Themethod of claim 12, wherein the first end surface is chemically treatedwith one of hydrofluoric acid and or a mixture of hydrofluoric acid withammonium fluoride.
 14. The method of claim 8, wherein the step offorming the recess on the first end surface includes etching the firstend surface.
 15. The method of claim 14, wherein the first end surfaceis etched with a substance having different etching velocities for thematerial in the core in the first piece and the material outside thecore in the first piece.
 16. The method of claim 8, further comprisingthe step of introducing a substance in the cavity, which substance isselected to increase a reflective capability of the cavity to lightwaves propagating along the optical fiber.
 17. The method of claim 16,wherein the step of introducing a substance in the cavity includesselecting the substance to have a refractive index different from therefractive index of the material in the first optical fiber piece and inthe second optical fiber piece surrounding the cavity.
 18. The method ofclaim 16, wherein the step of introducing a substance in the cavity isperformed by introducing a metal material in the cavity.
 19. The methodof claim 16, wherein the step of introducing a substance in the cavityis performed by introducing the substance in the recess before joiningthe first end surface to the second end surface.
 20. The method of claim8, further comprising the steps, before joining the first end surface tothe second end surface, of coating the first end surface with a layer ofa substance to increase a reflective capability of the cavity for lightwaves propagating in the optical fiber, and then grinding the coatedfirst end surface to a flat shape so that the layer is removedeverywhere on the end surface except in the recess.
 21. The method ofclaim 20, wherein the step of coating the first end surface with a layerof a substance includes selecting the substance to have a refractiveindex different from the refractive index of the material in the firstoptical fiber piece and in the second optical fiber piece surroundingthe cavity.
 22. The method of claim 20, wherein the step of coating asubstance over the first end surface is performed by coating with ametal material.
 23. The method of claim 8, wherein the first end surfaceis joined to the second end surface by melting the end surfaces to eachother so that a joint forms, and further comprising the step, afterjoining the first end surface to the second end surface, of heating thejoint to a temperature near the melt temperature of the materials in thefiber pieces for reducing the reflective capability of the producedreflector.
 24. An optical fiber network having a main line and severalbranch lines connected thereto, comprising:a monitoring unit connectedto the main line, a plurality of optical fiber pieces, each pieceincluding a fiber core, a cladding surrounding the fiber core and areflector, one optical fiber piece being placed in each branch line, thereflectors being located at different optical distances from themonitoring unit for reflecting light propagating from the monitoringunit along the optical fiber pieces back through the respective opticalfiber piece to the monitoring unit, the monitoring unit having means todetect light reflected from the reflectors and to indicate whenreflected light from a reflector is not detected, the reflector of atleast one optical fiber piece being formed by a cavity in the at leastone optical fiber piece.
 25. The optical fiber network of claim 24,wherein the cavity is located in the fiber core of the at least oneoptical fiber piece.
 26. The optical fiber network of claim 24, whereinthe cavity is located in a position so that light which propagates alongthe fiber is disturbed by the cavity, so that the light is partlyreflected.
 27. The optical fiber network of claim 24, wherein materialin the fiber core extends up to the cavity.
 28. The optical fibernetwork of claim 24, wherein that the cavity is substantially completelyenclosed by material of the fiber core.
 29. The optical fiber network ofclaim 24, wherein the cavity is shaped to have one of substantially alens shape and substantially a flattened ellipsoidal body shape, havingtwo opposite large curved or arched surfaces, and wherein material inthe fiber core extends up to at least the central part of one of theopposite large surfaces.
 30. The optical fiber network of claim 24,wherein a substance is disposed in the cavity for increasing thereflective capability of the cavity for light waves propagating in theoptical fiber piece.
 31. The optical fiber network of claim 30, whereinthe substance has a refractive index different from the refractive indexof material in the optical fiber piece enclosing the cavity.
 32. Theoptical fiber network of claim 30, wherein the substance hassubstantially a solid shape.
 33. The optical fiber network of claim 32,wherein the substance is coated only on one surface in the cavity. 34.The optical fiber network of claim 33, wherein the substance is coatedonly on that surface in the cavity which is directed towards a portionof the fiber core which is interrupted by the cavity.
 35. The opticalfiber network of claim 30, wherein the substance is a metal.